Although this cartoon pokes fun at scientific hypotheses, the concept of hypothesis is one of the most important in science. Scientific investigations discover evidence that helps science advance, and the purpose of scientific investigations generally is to test hypotheses. Finding evidence to support or disprove hypotheses is how science advances.

What Is a Scientific Hypothesis?

The word
hypothesis
can be defined as an "educated guess." For example, it might be an educated guess about why a natural event occurs. But not all hypotheses—even those about the natural world—are scientific hypotheses. What makes a statement a scientific hypothesis rather than just an educated guess? A scientific
hypothesis
must meet two criteria:

A scientific hypothesis must be testable.

A scientific hypothesis must be falsifiable.

A Scientific Hypothesis Must Be Testable

For a hypothesis to be testable means that it is possible to make observations that agree or disagree with it. If a hypothesis cannot be tested by making observations, it is not scientific. Consider this statement:

"There are invisible creatures all around us that we can never observe in any way."

This statement may or may not be true, but it is not a scientific hypothesis. That’s because it can’t be tested. Given the nature of the hypothesis, there are no observations a scientist could make to test whether or not it is false.

A Scientific Hypothesis Must Be Falsifiable

A hypothesis may be testable, but even that isn’t enough for it to be a scientific hypothesis. In addition, it must be possible to show that the hypothesis is false if it really is false. Consider this statement:

This statement is testable. If it is true, it is at least theoretically possible to find evidence showing that it’s true. For example, a spacecraft could be sent from Earth to explore the universe and report back if it discovers an inhabited planet. If such a planet were found, it would prove the statement is true. However, the statement isn’t a scientific hypothesis. Why? If it is false, it’s not possible to show that it’s false. The spacecraft may never find an inhabited planet, but that doesn't necessarily mean there isn’t one. Given the vastness of the universe, we would never be able to check every planet for life!

Both Testable and Falsifiable

Let’s consider one last example, which is illustrated in the
Figure
below
:

"Any two objects dropped at the same time from the same height will reach the ground at the same time (assuming the absence of air resistance)."

Is this statement testable? Yes. You could drop two objects at the same time from the same height and observe when they reach the ground. Of course, you would have to drop the objects in the absence of air to prevent air resistance, but at least such a test is theoretically possible. Is the statement falsifiable if it really is false? Again, the answer is yes. You can easily test many combinations of two objects and if any two objects do not reach the ground at the same time, then the hypothesis is false. If a hypothesis really is false, it should be relatively easy to disprove it.

Both the elephant and the boy are falling to the ground because of gravity. The force of gravity (F
grav
) is greater for the elephant than it is for the boy because the elephant is much more massive. Nonetheless, both of them will reach the ground at the same time (assuming they fall from the same height at the same time and there is no air resistance.)

Can You Prove a Hypothesis Is True?

If the hypothesis above about falling objects really were false, it is likely that this would be discovered sooner or later after enough objects had been dropped. It takes just one exception to disprove a hypothesis. But what if the hypothesis really is true? Can this be demonstrated as well? No; it would require testing all possible combinations of objects to show that they always reach the ground at the same time. This is impossible. New objects are being made all the time that would have to be tested. It’s always possible an exception would be found in the future to disprove the hypothesis. Although you can’t prove conclusively that a hypothesis is true, the more evidence you gather in support of it, the more likely it is to be true.

Summary

In science, a hypothesis is an educated guess that can be tested with observations and falsified if it really is false.

You cannot prove conclusively that most hypotheses are true because it’s generally impossible to examine all possible cases for exceptions that would disprove them.

Vocabulary

hypothesis
: Potential answer to a question that can be tested by gathering information.

Practice

Do the practice quiz on scientific hypotheses at the bottom of this Web page. Be sure to check your answers.